These studies employ Golgi and electron microscopic techniques to examine the postnatal differentiation of neurons in trigeminal pain pathways in trigeminal nucleus caudalis. These studies compare the maturity of neuronal cell types in newborn kittens with their adult counterparts and follow the developmental sequences in the formation of their dendritic and axonal arbors. These studies seek to understand the basic cellular mechanisms underlying postnatal dendritic retraction as well as dendritic and axonal elongation. They will also determine which components of the synaptic circuitry in trigeminal pain pathways through nucleus caudalis are present at birth and which develop postnatally.